While the Lighthouse implementation of the `ValidatorStore` does not really care about blobs, Anchor needs to be able to return different blobs from `sign_blocks` than what was passed into it, in case it decides to sign another Anchor node's block. Only passing the unsigned block into `sign_block` and only returning a signed block from it (without any blobs and proofs) was an oversight in #6705.
- Replace `validator_store::{Uns,S}ignedBlock` with `validator_store::block_service::{Uns,S}ignedBlock`, as we need all data in there.
- In `lighthouse_validator_store`, just add the received blobs back to the signed block after signing it.
- Create trait `ValidatorStore` with all functions used by the `validator_services`
- Make `validator_services` generic on `S: ValidatorStore`
- Introduce `LighthouseValidatorStore`, which has identical functionality to the old `ValidatorStore`
- Remove dependencies (especially `environment`) from `validator_services` and `beacon_node_fallback` in order to be able to cleanly use them in Anchor
